1. Field of the Invention
The present invention relates to a projection type display device and more particularly relates to a single-panel projection type display device for use to conduct a color display.
2. Description of the Related Art
A projection type color image display device using a liquid crystal display panel has been known in the art. A projection type color liquid crystal display device should be provided with a light source separately because the liquid crystal display panel thereof does not emit any radiation spontaneously. However, a projection type color liquid crystal display device has a number of advantages over a projection type cathode-ray tube (CRT) display device. Specifically, a projection type color liquid crystal display device realizes a broader color reproducible range, is easier to carry due to its smaller size and lighter weight, and does not require any convergence correction because a device of that type is not affected by geomagnetism. Thus, further development of devices of that type is awaited.
Projection type color liquid crystal display devices are classifiable into a three-panel type and a single-panel type according to their methods of displaying an image. Specifically, a projection type color liquid crystal display device of the three-panel type uses three liquid crystal display panels (which will also be referred to as “liquid crystal display elements” herein) that are respectively provided for the three primary colors of light. On the other hand, a projection type color liquid crystal display device of the single-panel type uses just one liquid crystal display panel to display an image. The three-panel projection type liquid crystal display device of the former type includes an optical system and three liquid crystal display panels. The optical system is provided to separate white light, emitted from a light source, into three light beams representing the three primary colors of red, green and blue (which will be herein referred to as “color light beams”) and then direct these color light beams toward the three liquid crystal display panels. The three liquid crystal display panels are provided to form an image by independently controlling these color light beams. That is to say, the color light beams are modulated by these three liquid crystal display panels, respectively. By optically superimposing these modulated color light beams one upon another on the screen, an image can be displayed thereon in full colors.
A display device of this three-panel type realizes not only highly efficient use of the light from the light source, but also high color purity for the image displayed. However, the three-panel type needs the optical system for directing the color light beams toward the respective liquid crystal display panels and another optical system for synthesizing together the color light beams that have been modulated by the panels. As also described above, this type requires three liquid crystal display panels. Accordingly, compared to the single-panel type, the three-panel type needs much more complicated optical systems and a far greater number of components. Thus, the three-panel type contributes to cost and size reductions much less than the single-panel type.
In contrast, a projection type liquid crystal display device of the latter single-panel type needs just one liquid crystal display panel and much simpler optical systems, thus contributing to cost reduction advantageously. Accordingly, the single-panel type can be used effectively for a small-sized projection system. Specifically, a known single-panel projection type liquid crystal display device may use a liquid crystal display panel including color filters that have been formed in a mosaic or striped pattern for the three primary colors. The single-panel type gets the incoming light modulated by the liquid crystal display panel, and then projects the modulated light through a projecting optical system onto a screen. A display device of this type is disclosed in Japanese Laid-Open Publication No. 59-230383, for example.
However, the display device of the single-panel type can utilize only about one-third of the incoming light for the display purposes because the light is absorbed into, or reflected from, the color filters unintentionally. That is to say, supposing a light source for emitting light of the same brightness is used for both the single-panel and three-panel types, the brightness of the image formed on the screen by the single-panel type including the color filters decreases to about one-third of that of the image formed thereon by the three-panel type.
To avoid the decrease in brightness of the image on the screen, the light source may have its brightness increased. However, particularly when a display device of the single-panel type is intended to be a household appliance, the brightness of the light source should not be increased. This is because the power dissipation thereof would increase disadvantageously by doing so. Also, where the color filters used absorb light, the energy of the light that has been absorbed into the color filters changes into heat. Accordingly, if the light source were brightened in that case, not only excessive rise in temperature of the liquid crystal display panel but also accelerated fading of the color filters should be inevitable. Thus, to heighten the value of a projection type color image display device of the single-panel type, it is a key problem to be solved how to utilize the light more efficiently without brightening its light source.
To overcome this problem, a single-panel projection type color image display device proposed tries to utilize the incoming white light more efficiently by getting the light separated into multiple color light beams by dichroic mirrors, not color filters. See Japanese Laid-Open Publication No. 4-60538, for example.
As shown in FIG. 14, the projection type color image display device separates white light, which has been emitted from a white light source 101, into three light beams representing red (R), green (G) and blue (B) by using three dichroic mirrors 104R, 104G and 104B, respectively. These dichroic mirrors 104R, 104G and 104B are disposed to define mutually different angles with the direction in which the white light travels. The three light beams representing R, G and B will be herein referred to as “R light beam”, “G light beam” and “B light beam”, respectively. The R, G and B light beams, into which the white light has been separated by the dichroic mirrors 104R, 104G and 104B, are incident onto a microlens array 105 at mutually different angles. The microlens array 105 is disposed on a surface of a liquid crystal display panel 107.
FIG. 15 illustrates a configuration for the liquid crystal display panel 107 of the projection type color liquid crystal display device shown in FIG. 14. The liquid crystal display panel 107 has a structure in which a liquid crystal layer 107c is sandwiched between a pair of transparent substrates 107a and 107b that faces each other. On the surface of the transparent substrate 107a that is opposed to the liquid crystal layer 107c, a driver including pixel electrodes, thin-film transistors (TFTs) and bus lines, an alignment layer (not shown) and a black matrix (BM) 111 are provided. The BM 111 is formed as an opaque layer for preventing the bus line regions from being exposed to the incoming light beams.
The microlens array 105 is an array of microlenses 106, each having a size corresponding to three pixels of the liquid crystal display panel 107. The light beams in the respective colors that have been incident onto the microlens array 105 at mutually different angles are distributed and irradiated to their associated pixels of the liquid crystal display panel 107 in accordance with their incident angles. In other words, the R, G and B light beams are converged by each of these lenses 106 onto their associated set of three pixels. At the three pixels on which the R, G and B light beams have been converged, the light modulating functions (more specifically, the orientation states) of their associated parts of the liquid crystal layer 107c are controlled independently in accordance with image signals. The light that has been subjected to the desired modulation in this manner is projected onto a screen 110 by way of a field lens 108 and a projection lens 109 that are provided in front of the liquid crystal display panel 107.
This projection type color image display device uses the dichroic mirrors 104R, 104G and 104B and the microlens array 105 to make the light beams in the three primary colors incident onto mutually different pixels, and needs no color filters that absorb light. Accordingly, a display device of this type can utilize the incoming light more efficiently and can display a brighter image.
The liquid crystal display panel 107 is also provided with the black matrix (BM) 111 for the respective pixels so as to prevent the TFTs and bus line regions from being exposed to the incoming light beams. Each pixel has a so-called “pixel opening” 112, which is not covered with the BM 111 and through which the incoming light can pass. The ratio of the size of the pixel opening 112 to the pixel size is normally called an “aperture ratio”.
Without the microlens array 105, the incoming light would be incident as parallel light onto the liquid crystal display panel 107. In that case, parts of the incoming light would be cut of f by the BM 111 and could not contribute to the display anymore. As a result, the incoming light could not be utilized so efficiently. In contrast, this projection type color image display device can get the incoming light collected onto the pixel openings 112 by the microlens array 105. Accordingly, a greater quantity of light can pass through the liquid crystal display panel 107 and a brighter image can be projected onto the screen 110.
Suppose three liquid crystal display panels included in the three-panel projection type liquid crystal display device and one liquid crystal display panel included in this single-panel projection type liquid crystal display device each have the same resolution (or the same number of pixels). In that case, the resolution of a full-color image formed on the screen by the three-panel type will be three times as high as that of an image formed thereon by the single-panel type. The reason is as follows. A display device of the three-panel type forms a color image on the screen by synthesizing together the R, G and B light beams that have gone out of the three liquid crystal display panels, respectively. Accordingly, a full-color image, having the same number of pixels as that of the pixels of each liquid crystal display panel, can be displayed on the screen. In the single-panel type on the other hand, one pixel of the liquid crystal display panel is associated with just one of R, G and B. Thus, a full-color image displayed on the screen by the single-panel type has a pixel number just one-third as large as that of the liquid crystal display panel.
FIG. 16 illustrates a situation where a display panel having the same resolution as that of the three display panels of the three-panel type is used for a single-panel projection type liquid crystal display device. Each of the R, G and B pixels has an approximately square planar shape and each microlens 106b has a size corresponding to the total size of the three R, G and B pixels. In this case, each microlens 106b has a horizontally elongated planar shape, of which the horizontal size is three times as long as its vertical size. In FIG. 16, R, G or B does not indicate the color of a color filter provided for its associated pixel but the color of the light beam to be converged on that pixel through its associated microlens.
On the other hand, if a liquid crystal display panel, having three times as large a number of pixels as that of the three display panels of the three-panel type, is provided for the single-panel type, even the single-panel type achieves the resolution of the three-panel type. FIG. 17 illustrates pixels and microlenses in a liquid crystal display panel for a high-resolution, single-panel display device. As shown in FIG. 17, a square picture element (i.e., a minimum unit for displaying a full-color image) is made up of three pixels, through which the R, G and B light beams pass, respectively. Also, multiple R, G and B pixels, each of which has a horizontal size one-third as long as its vertical size, are arranged to form a striped pattern. Each of the condensing microlenses 106a is provided for its associated set of three R, G and B pixels. In this case, each microlens 106a has an approximately square planar shape.
However, if the three R, G and B pixels are provided for one pixel region of the three-panel type as shown in FIG. 17, additional bus line regions are required to control the two extra pixels separately. That is to say, since the number of bus line regions and the number of switching elements such as TFTs for controlling the pixels increase, the production yield of the display device decreases and the fabrication cost thereof increases instead. Thus, to fabricate a liquid crystal display panel more easily at a lower cost, the arrangement shown in FIG. 16 is more advantageous over that shown in FIG. 17. Accordingly, if a high resolution is not needed or if the resolution of an image on the screen can be increased without increasing the number of pixels of the liquid crystal display panel, a liquid crystal display panel such as that shown in FIG. 16, having the same pixel arrangement as that of the three liquid crystal display panels for the three-panel type, is preferably used.
In addition, if the arrangement shown in FIG. 17 is adopted, then the area of each light-transmitting pixel opening decreases to less than one-third of the area of each square pixel shown in FIG. 16. As a result, the aperture ratio decreases.
However, even though a display device having the pixel arrangement shown in FIG. 17 has a lower pixel aperture ratio than a display device having the pixel arrangement shown in FIG. 16, an image projected onto the screen by the former type may actually be brighter than an image formed thereon by the latter type. That is to say, even if the pixel aperture ratio is set relatively high as shown in FIG. 16, an image having sufficiently high brightness may not be obtained.
One imaginable reason why the image projected gets dark even though the pixel aperture ratio is relatively high may be inappropriate incidence of outgoing light onto the projection lens. An F number is one of the indices representing the performance of a projection lens, and is obtained by dividing the focal length f of the lens by the diameter D of the light-transmitting area (i.e., entrance pupil) of the lens. Generally speaking, the smaller the F number of a projection lens, the greater the diameter D (or the area) of its entrance pupil is. That is to say, its light-receiving area has a broader angle and a greater quantity of light can be used to form a projected image. As a result, a brighter image can be formed at a small F number.
However, a projection lens always has to focus any light coming from a predetermined pixel onto an intended point on the screen, no matter which part of the projection lens that light has passed through. For that reason, as the F number of a projection lens decreases, it becomes more and more difficult, and takes an increasingly high cost, to make the projection lens as intended. Thus, the F number of a projection lens actually used must not be decreased excessively but needs to fall within a predetermined range.
In the projection type color image display device shown in FIG. 14, the light beams, which have gone out of the liquid crystal display panel 107, enters the projection lens 109 by way of the field lens 108. Each of these light beams is incident onto a predetermined point in the entrance pupil of the projection lens 109 in accordance with the angle at which the light beam has gone out of the liquid crystal display panel 107. If the projection lens 109 has no opaque portions, then the entrance pupil of the lens 109 corresponds to its cross section taken substantially vertically to the direction in which the light is incident. If a light beam has gone out of the liquid crystal display panel 107 in a direction that is substantially parallel to the optical axis of the lens 109, then the light beam passes through approximately the center of the pupil of the lens 109. On the other hand, if a light beam going out of the liquid crystal display panel 107 defines a large angle with the optical axis of the lens 109, then the light beam goes away from the lens 109. As described above, the F number of a projection lens has a lower limit. Accordingly, if a light beam going out of the liquid crystal display panel 107 defines an angle greater than the maximum allowable angle defined by the lowest possible F number, then the light beam will deviate from the entrance pupil of the projection lens 109. As a result, such a light beam cannot reach the screen 110, i.e., cannot contribute to the image projection. That is to say, even if each pixel of a liquid crystal display panel has a high aperture ratio and a large quantity of light goes out of the liquid crystal display panel, the angle defined by the outgoing light should not be too large to increase the brightness of an image formed on the screen.
Conversely, if a light beam going out of the liquid crystal display panel 107 defines an angle smaller than the minimum allowable angle obtained from the lowest possible F number, then the light beam will not be incident on the entire entrance pupil of the projection lens 109 (i.e., there is some margin for the F number of the projection lens). In such a situation, a bright image cannot be displayed, either. To increase the brightness of the image projected, the light beam going out of the liquid crystal display panel 107 should have its angle increased by decreasing the focal length of the microlenses for use to converge the incoming light onto the pixels, for example. Then, even a light beam having a low degree of parallelism with respect to the principal ray that has been emitted from the light source can also pass through the opening of a pixel. As a result, a brighter image can be projected.
As can be seen, to obtain a brightest possible image, a light beam should go out of the liquid crystal display panel at such an angle as to enter the entire entrance pupil of the projection lens having a limited size.
FIGS. 18 and 19 illustrate how the light going out of the liquid crystal display panel 107 is distributed in the entrance pupil of the projection lens 109 (corresponding to a cross section of the projection lens 109 in this case) when the pixels of the liquid crystal display panel 107 and the microlenses 106b or 106a are arranged as shown in FIGS. 16 and 17, respectively. The R light beam is vertically incident onto the microlens array 105 as shown in FIGS. 14 and 15, and therefore enters the center of the circular pupil plane 113 as shown in FIGS. 18 and 19. On the other hand, the B and G light beams each define an angle with respect to the R light beam, and therefore the point of incidence of the B or G light beam onto the pupil plane 113 deviates rightward or leftward from the center of the pupil plane 113 as shown in FIGS. 18 and 19. It should be noted that any light beam going out of the liquid crystal display panel 107 always passes through some area of the lens pupil plane 113 depending on its color as shown in FIGS. 18 and 19, no matter which pixel of the panel plane the light beam has passed through. This is because the light outgoing from the entire panel plane of the liquid crystal display panel 107 is collected by the field lens 108 onto the projection lens 109.
Each area of the entrance pupil, on which one of the three color light beams is incident, has its shape determined by the shape of its associated microlens, the opening shape of its associated pixel and the focal length of the microlens, for example. As can be seen from FIG. 19, if the pixels and the microlenses 106a are arranged as shown in FIG. 17, the R, G and B light beams are each distributed in a substantially square shape. On the other hand, if the pixels and the microlenses 106b are arranged as shown in FIG. 16, horizontally elongated rectangular R, G and B light beams are distributed so as to partially overlap with each other as shown in FIG. 18. In FIG. 18, an area in which the R and B light beams overlap with each other is identified by M (magenta), an area in which the R and G light beams overlap with each other is identified by Y (yellow), and an area in which the R, G and B light beams overlap with each other is identified by W (white).
When the arrangement shown in FIG. 16 is adopted, the image projected darkens in spite of the high pixel aperture ratio. This phenomenon occurs because the R, G and B light beams have horizontally elongated distributions on the entrance pupil of the projection lens due to the shape of its associated microlens. As a result, unusable areas U, on which no light beams are incident, are formed in the upper and lower parts of the projection lens. In such a situation, the projection lens cannot be used efficiently enough, and just a small percentage of light is usable for display purposes if the light has a low degree of parallelism with respect to the principal ray. Accordingly, a bright image cannot be projected onto the screen.
As can be seen, it is difficult for the conventional projection type display device to obtain a brightly projected image by using a liquid crystal display panel including substantially square pixels as shown in FIG. 16. However, to fabricate a liquid crystal display panel more easily at a lower cost, the arrangement shown in FIG. 16 is more advantageous over that shown in FIG. 17.